The invention relates to a hydraulic threaded bolt clamping device for tightening and releasing large screws and a method for tightening such screws by means of the threaded bolt clamping device that comprises a pressure medium supply to the threaded bolt clamping device, a pressure medium connector on the threaded bolt clamping device, a cylinder resting against a machine part that is to be clamped by means of a threaded bolt and a nut, at least one periodically pressure-loaded piston that rests against a threaded bushing threaded onto a threaded end of the threaded bolt, and a control device for actuating the pressure medium supply to the threaded bolt clamping device.
In order to obtain precise information of the screw prestress, it is known to directly measure the achieved screw strain because the screw strain, as a result of Hook's law, is directly proportional to this screw prestress. This strain measurement can be realized in different ways.
According to DE 28 46 668 A1 the threaded bolt to be clamped can be provided with a through bore over its entire length and a measuring rod can be inserted into this bore; the length of the measuring rod should match essentially the length of the threaded bolt and the measuring rod does not participate in the strain of the threaded bolt so that the strain of the threaded bolt is determined by the length difference between the measuring rod and the threaded bolt.
This measuring method is complex because each threaded bolt must be provided with a through bore and the measuring rod must be inserted into the bore and connected to a suitable measuring device for each clamping action of the threaded bolt.
In a hydraulic threaded bolt clamping device disclosed in DE 101 45 847 of the same applicant, a measuring method is employed that resides in that the threaded bolt strain is to be determined based on measuring the rotation angle of the nut because, based on the respective pitch of the thread, there is a constant ratio between the rotation angle of the nut and the threaded bolt strain. In this measuring method, it is required to first tighten the nut to a point where all components to be clamped by the threaded bolt are positioned relative to one another without play and to take into account the rotation angle of the nut only from the thus determined joining point on.
The two known measuring methods enable the determination of the strain of the threaded bolt that is caused by pressurizing the hydraulic threaded bolt clamping device with satisfactory precision and enable an extensive automation of the tightening action of large screws, but they require an increased apparatus expenditure for this purpose.